1. Field of the Invention
The present disclosure relates to an apparatus and method for sound field control, and in particular, the present disclosure relates to a technique suitable for use in a sound field control apparatus for adjusting or creating a space (sound field) where there is audio reproduced by an audio system.
2. Description of the Related Art
There have been provided many sound field control apparatuses for adjusting or creating a space (sound field) where there is audio reproduced by an audio system. Techniques for recreating a sound field just like in a real concert hall or movie theater through an audio system intended for home use have also been developed.
Most sound field control apparatuses proposed so far control a sound pressure level alone in a space. However, controlling a sound pressure level alone at a fixed point cannot control the velocity of particles as the flow of air upon propagation of a sound wave. It may produce a feeling of strangeness in the direction in which sound comes. Techniques for controlling an acoustic intensity corresponding to the product of a sound pressure level and a particle velocity or an acoustic impedance corresponding to the ratio of a sound pressure level to a particle velocity have also been proposed.
Controlling the acoustic intensity or acoustic impedance indirectly controls the sound pressure level and the particle velocity. The sound pressure level and the particle velocity are not necessarily controlled to desired states. For example, in a sound field control apparatus mounted on an in-vehicle audio system, it is desirable to create a sound field so that reproduced sound is equally audible by all persons sit in a vehicle interior. However, it is difficult to realize such a sound field by conventional methods for acoustic intensity control and acoustic impedance control.
Acoustic intensity control is intended to control acoustic intensities in directions excluding one direction so that the acoustic intensities approach to zero. Accordingly, an acoustic intensity in the one direction cannot be controlled to a desired value. If control conditions are not good, the direction of acoustic intensity flow may be opposite to a desired direction.
FIGS. 7A and 7B illustrate a sound pressure distribution and a particle velocity distribution when acoustic intensities were controlled in a predetermined space. The predetermined space is obtained by simulating a space in a vehicle interior. The x1-axis direction (corresponding to the length direction of the vehicle interior) is set to 2 m, the x2-axis direction (corresponding to the width direction thereof) is set to 1.3 m, and the x3-axis direction (corresponding to the height direction thereof) is set to 0.8 m.
As for the acoustic intensity control, for example, the acoustic intensity in the x2-axis direction (the width direction of the vehicle interior) is controlled at zero, so that sound pressure levels in the x2-axis direction can be substantially equalized, as illustrated in the sound pressure distribution of FIG. 7A. However, sound pressure levels in the x1-axis direction (the length direction of the vehicle interior) cannot be equalized. Referring to FIG. 7A, sound pressure levels are too high in positions corresponding to the windshield of a vehicle and a headrest of a rear seat. On the other hand, sound pressure levels are too low in positions corresponding to a headrest of a front seat. Furthermore, air particles flowed from a rear portion of the vehicle interior to a front portion thereof, as illustrated in FIG. 7B.
Acoustic impedance control is intended to control an acoustic impedance in one direction so that the acoustic impedance is equalized to the characteristic impedance of air in order to cancel out reflected sound in the one direction. In this case, acoustic impedances in other directions cannot be controlled to desired values. If control conditions are not good, the direction of acoustic impedance flow may be opposite to a desired direction.
FIGS. 8A and 8B illustrate a sound pressure distribution and a particle velocity distribution when acoustic impedances were controlled in the same space as that in FIGS. 7A and 7B. As for the acoustic impedance control, for example, the acoustic impedance in the x2-axis direction (the width direction of the vehicle interior) is controlled so that the acoustic impedance is equalized to the characteristic impedance of air, so that sound pressure levels in the x2-axis direction can be substantially equalized, as illustrated in the sound pressure distribution of FIG. 8A. However, sound pressure levels in the x1-axis direction (the length direction of the vehicle interior) cannot be equalized. Accordingly, sound pressure levels are too high in positions corresponding to the windshield of the vehicle and the headrest of the rear seat and sound pressure levels are too low in positions corresponding to the headrest of the front seat in a manner similar to that illustrated in FIG. 7A. Furthermore, the flow of air particles from the front portion of the vehicle interior to the rear portion and that from the rear portion to the front portion were mixed, as illustrated in FIG. 8B.
There has been proposed a technique of obtaining the relationship between a temporal change in sound pressure level and that in air particle velocity and the relationship between a spatial change in sound pressure level and that in air particle velocity, obtaining a sound pressure level at a specified position in a space on the basis of the obtained relationships, and outputting the obtained sound pressure level (refer to Japanese Patent No. 3863306, for example).
According to the conventionally proposed control techniques, a sound pressure level and an air particle velocity are indirectly controlled. Disadvantageously, control performance is not sufficiently delivered when these techniques are applied to, for example, an in-vehicle audio system.
According to the technique disclosed in Japanese Patent No. 3863306, a sound pressure level alone at a specified position is obtained on the basis of the relationships between changes in sound pressure level and those in air particle velocity. The technique is not intended to correct sound pressure levels and air particle velocities in an acoustic space to desired characteristics. New techniques are desirable to correct sound pressure levels and air particle velocities in the acoustic space to desired characteristics.